Wireless Voice Best Practices (in a pre-6GHz world)

Posted byMatthew McClearnPosted inUncategorizedTags:,

For the almost 3 years now I’ve been working for Wireless VOIP client. In that time I have troubleshooted over 1300 support cases related to our clients’ networks and our devices’ interactions with it. Here is a culmination of what I’ve learned.

QoS

  • Ensure wireless QoS is in place and maintaining UP of 6 end-to-end
    • If the client doesn’t tag its frames then a QoS policy will need to be put in place on the wireless to tag frames at the AP
    • Client QoS tagging can be verified in a packet capture
  • Ensure wired QoS in in place and maintaining DSCP of 46 (EF).
    • Wireless QoS only affects the wireless frame over the air. Wired QoS ensures that the frame has priority as it traverses the wired LAN

Beware of (Call) Admission Control

  • Call Admission Control is an mechanism, utilized by the wireless infrastructure, that assigns and dedicates individual slots of “airtime” and bandwidth to VOIP calls.
  • CAC utilizes a protocol known as TSPEC (Traffic Specification) in which clients request available VOIP call slots based on resources needed (bandwidth, frame size, etc.) and what is available.
  • If your client does not support TSPEC, CAC is enabled, then they may not be able to negotiate the desired QoS they need for their calls.

Avoid 2.4GHz if possible.

  • This may not be possible for all customers.
  • Being there are only 3 non-overlapping channels to use in 2.4GHz, this makes controlling Co-Channel Interference while supporting client density a daunting task for any Network Engineer.
  • Co-Channel Interference increases contention, thereby increasing the time stations need to wait for the channel to be free.
  • Voice Codecs require a steady stream of audio to maintain a quality user experience.

Ensure time spent scanning for better AP at roam time is as quick as possible

  • If possible, client should be configured to only scan channels used by the network
  • DFS channels require passive scanning (wait before transmit), thereby increasing scan time significantly over active scanning (probe-request / probe-response) of non-DFS channels. DFS channels should be limited as much as possible when deploying voice, however, the implementation of 802.11k will dramatically cut down the overall time spent passively scanning a channel plan utilizing DFS channels.
  • Client and network should have 802.11k enabled to allow for client to get list of nearby APs to current associated AP. Client will then scan these channels first when roaming as opposed to entire channel list.

Ensure roam times are quick once AP is chosen

  • If client is using EAP authentication, then RADIUS communications with an Authentication Server will be needed when roaming.
  • “Fast Roaming” protocols like CCKM, OKC, and 802.11r (Fast Transition (FT)) cache encryption keys thereby bypassing the need for further RADIUS communications when roaming. CCKM is Cisco Proprietary and is being deprecated in the new 9800 controller software releases. OKC is vendor-dependent and not widely supported. 802.11r (FT) is the new standard for “Fast Roaming”.
  • PSK authentication roams do not include RADIUS communication and therefore do not need to use “Fast Roaming” protocols.

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